THE SUN SHOWS SIGNS OF LIFE
NASA
After more than two years of few sunspots, and even fewer solar
flares, the Sun is finally showing signs of life. During October,
five sunspot groups were observed. That may not sound like much, but
in a year with record-low numbers of sunspots and long stretches of
spotlessness, five is significant and represents a real increase in
solar activity. Even more significant is the fact that four of them
belonged to Solar Cycle 24, the long-awaited next instalment of the
Sun's 11-year solar cycle. Solar Cycle 23 peaked in 2000 and has
since decayed to low levels. Meanwhile, the new Cycle 24 has been
slow to get started. 2008 is a year of overlap, with both cycles
weakly active at the same time. From January to September, the Sun
produced a total of 22 sunspot groups, 18 of which belonged to the old
Cycle 23. October added five more, but this time four belonged to
Cycle 24.
At first glance, old- and new-cycle sunspots look the same, but they
are not. To tell the difference, solar physicists check two things:
a sunspot's heliographic latitude and its magnetic polarity.
(1) New-cycle sunspots always appear at high latitude, while old-cycle
spots cluster around the sun's equator. (2) The magnetic polarity of
new-cycle spots is reversed compared to that of old-cycle spots. Four
of October's five sunspot groups satisfied those two criteria for
membership in the new cycle.
MESSENGER SHOWS MORE OF MERCURY
NASA
The Messenger spacecraft has passed by Mercury for the second time
this year and imaged a further 30% of the previously unseen part of
the surface. When combined with data from the first fly-by and from
Mariner 10, the latest coverage means that we have now seen about 95%
of the planet. Cameras took more than 1,200 pictures of the surface,
while the laser altimeter obtained profiles of the topography. The
comparison of the new magnetospheric observations with those from the
spacecraft's first flyby in January provides new insight into
Mercury's internal magnetic field and reveals new features of its
magnetosphere.
Previous fly-bys by Messenger and Mariner 10 provided data only about
Mercury's eastern hemisphere. The most recent fly-by observed the
western hemisphere, and showed that the planet's magnetic field is
highly symmetrical. The probe's laser altimeter allowed topographic
measurements to be correlated with images for the first time at high
resolution. A spectrometer observed Mercury's thin atmosphere, known
as its exosphere. The instrument searched for emissions from sodium,
calcium, magnesium, and hydrogen atoms. Observations of magnesium
were the first detection of that element in Mercury's exosphere. Now
that Messenger has imaged more than 80% of Mercury, it is clear that,
unlike the Moon and Mars, Mercury's surface is more homogeneously
ancient and heavily cratered, with large extents of younger volcanic
plains lying within and between giant impact basins.
ORBITER SHOWS DETAILS OF A WETTER MARS
NASA
The Mars Reconnaissance Orbiter has observed a new category of
hydrated minerals spread across large regions of Mars. Deposits of
such minerals indicate where and when water was present. The new
observations have identified hydrated silica, commonly known as opal,
whose presence suggests that water may have existed as recently as
2 billion years ago, a billion years later than scientists had
previously supposed. Until now, spacecraft orbiting Mars had observed
only two major groups of hydrated minerals, phyllosilicates and
hydrated sulphates.
Clay-like phyllosilicates formed more than 3.5 billion years ago where
igneous rock came into long-term contact with water. During the next
several hundred million years, until approximately 3 billion years
ago, hydrated sulphates formed from the evaporation of salty and
sometimes acidic water. The newly discovered opaline silicates are
the youngest of the three types of hydrated minerals. They formed
where liquid water altered materials created by volcanic activity or
meteorite impact on the Martian surface. One such location is the
large Martian canyon system Valles Marineris.
SPITZER STUDIES COMET HOLMES
NASA
When Comet Holmes unexpectedly erupted in 2007, astronomers around the
world turned their telescopes toward the spectacular event. People
would naturally like to know why the comet had suddenly exploded, but
observations recently reported from the Spitzer space telescope do not
answer the question, showing only oddly-behaving streamers in the
shell of dust surrounding the nucleus. The Comet Holmes explosion
gave us a rare glimpse of material from the inside of a comet nucleus,
and the data do not look like anything we typically see in comets.
Every six years, Comet Holmes heads inwards towards the Sun from the
distance of Jupiter's orbit. Usually it travels without incident, but
twice in the last 116 years, in 1892 November and 2007 October, it
suffered an explosion as it approached the asteroid belt, and
brightened a millionfold overnight. Spitzer observed the comet last
November and again in March. Its infrared spectrograph gave
indications of the composition of Holmes' solid interior. The
November observations showed a lot of fine silicate dust and materials
similar to those seen around other comets where grains have been
treated violently, as in the Deep Impact mission, which smashed a
projectile into Comet Tempel 1. Comet dust is very easily destroyed
and it is thought that the fine silicates are produced in violent
events by the destruction of larger particles originating inside the
comet nucleus. When Spitzer observed the same portion of the comet
again in March, the fine-grained silicate dust was gone and only
larger particles were present, so there seems to be only a small
window of time for studying comet dust after a violent event. Comet
Holmes not only has unusual dusty components, but it also does not
look like a typical comet. Pictures taken from the ground shortly
after the outburst showed streamers in the shell of dust surrounding
the comet. Scientists suspect that they were produced after the
explosion by fragments escaping the comet's nucleus. In 2007 November
the streamers pointed away from the Sun, which agreed with the idea
that radiation from the Sun was pushing the fragments straight back.
However, when Spitzer imaged the same streamers in March, they were
still pointing in the same direction even though the comet had moved
and sunlight was arriving from a different direction.
EPSILON ERIDANI HAS TWO ASTEROID-LIKE BELTS
Harvard-Smithsonian CfA
The Spitzer infrared telescope has observed Epsilon Eridani, a star
that is younger and slightly smaller and cooler than the Sun and, at a
distance of only 10.5 light-years, is the ninth-closest star. The
star proves to have three rings of cool material that emit radiation
only in the infrared. It is conjectured that the radiation arises
from bands of asteroid-like objects. The innermost band is about 3
astronomical units from the star, just like the asteroid belt in the
Solar System, and judged by its brightness it contains an amount of
material comparable with that in our asteroid belt. The second ring
seen in the infrared is at about 20 astronomical units from the star
(about the distance of Uranus from the Sun) and appears to hold about
20 times as much material as the first. A third ring, that has
been observed previously, extends from about 35 to 100 astronomical
units from Epsilon Eridani. A possibly analogous band in the Solar
System is called the Kuiper Belt; however, judged from its brightness,
Epsilon Eridani's outer ring holds about 100 times more material than
ours. The Spitzer data show gaps between the successive rings
surrounding Epsilon Eridani. Such gaps might be explained by the
presence of planets that constrain the rings gravitationally, just as
the moons of Saturn constrain its rings.
MARS ROVERS UPDATE
New Scientist
The arrival of spring in the southern hemisphere of Mars is reviving
the two venerable Mars rovers as deepening autumn in the arctic north
freezes the Phoenix lander, which failed to wake up on November 2.
After hibernating for the winter on the northern edge of a plateau,
the 'Spirit' rover moved uphill in October to collect more sunlight.
On the other side of the planet, the 'Opportunity' rover, which
climbed out of a large crater called Victoria at the end of August,
has completed the first month of a 12-kilometre trek towards an even
bigger crater called Endeavour. That journey is expected to take more
than two years. Designed to last only 90 days, the two rovers have
survived for nearly five years.
Once it started moving, Spirit was able to climb slopes of up to 30°.
However, two weeks of movement has not dislodged the dust that limits
the power generated by its solar arrays. Winds have blown dust off
Opportunity's solar panels, so they generate more than 25 watts and
allow the rover to move faster, but it must navigate carefully across
ground that includes soft dunes where it could get stuck. The rover
can travel up to 100 metres in an hour, but it can see only 20 to 30
metres ahead. That means that it has to stop regularly to send
pictures of the prospect so that controllers can pick out a safe
path. Both rovers will have to be parked during an interruption in
communications from November 29 to December 15, when the Sun will be
between Mars and the Earth.
Topic: Mid November Astronomy News (Read 1335 times)